Sometimes, I come along a question here on Academia that I find interesting, or that I can strongly relate with. When this happens I tend to upvote it: I think of my upvote as saying "I consider this a good question" and/or "I would also like to have an answer for it". But sometimes these questions are (clearly) not a good fit for the site. So my question is: should I think about an upvote as "good question AND on-topic"? Am I creating more difficulties by upvoting an off-topic question? Or it just doesn't matter at all?
I'd say: we have the tools to make this statement explicit. Up-vote the question and leave a comment saying that you up-voted because it's a good question, but you think it is not on topic for reason X.
If it needs to be migrated, that is probably the end of it. If it has a different problem (e.g., being opinion-based), then you might also consider proposing edits to make it a more answerable question.
It is your vote and you should vote as you see fit. I think of up votes as saying this is a good question that is a good fit for our community. I only like rewarding people for doing things that help our community. There is also a small reduction of work if closed questions are not up voted since closed questions with negative vote totals are automatically deleted. Questions with a score of zero or more require high rep users to vote to delete them.